Shale reservoirs are hydrocarbon reservoirs formed in a shale formation. It can be difficult to produce the hydrocarbons from shale reservoirs because the shale formation is of low porosity and low permeability. This means that when a well is drilled into the formation, only those fluid hydrocarbons in proximity to the well are produced, as the other hydrocarbons further away from the well have no easy path to the well through the relatively impermeable rock formation.
A typical production system is illustrated in FIG. 1, in which a subterranean shale formation 1 is exploited. A reservoir of liquid hydrocarbons is at a certain depth 2. These are exploited by drilling a horizontal well 3 from a production facility 4 located at the surface. A horizontal well 3 allows a greater length the well to be in contact with the reservoir 2. Note also that substantially vertical wells may be used.
The following discussion refers to shale reservoirs, but the same techniques apply to hydrocarbon recovery from other low porosity, low permeability formations. In order to improve hydrocarbon recovery from shale reservoirs, the shale around the well 3 is often hydraulically fractured, as illustrated in FIG. 2. In the example of FIG. 2, the well is located in a shale (or other low porosity/low permeability) formation 2 bearing hydrocarbons, and surrounded above and below by a cap rock formation 5 and the base rock formation 6 respectively. Once a fracture has been hydraulically induced, it is typically held open using a proppant.
Hydraulic fracturing involves propagating fractures 7 through the shale formation 2 using a pressurized fluid. These fractures create conduits in the low permeability shale formation. Hydrocarbon fluids can then migrate through the conduits towards the well 3. In this way, recovery of hydrocarbons from the reservoir is improved because hydrocarbons that would not previously be able to reach the well now have a path to the well and can be produced.
Hydraulic fracturing leads to a high initial production of hydrocarbons trapped in the shale reservoir. However, this high initial production quickly tails off to a value of typically between 10 and 20% of the initial production rate. Over the lifetime of a shale reservoir well, the well may produce an average of 400-500 BOE (barrels of oil equivalent) per day, peaking in the initial stages at around 1,500 BOE per day.
Furthermore, hydraulic fracturing only leads to a part of the hydrocarbons trapped in the shale being produced. This is because the pattern of the fractures created during the hydraulic fracturing process does not provide access to the entire pore space of the shale formation. Some regions of the shale reservoir are therefore out of reach by the production well 3 due to the low permeability of the shale formation. The production rate also drops because the main driving force pushing hydrocarbons towards the well is fluid expansion due to pressure depletion. The pressure reduces as the hydrocarbons are produced.